[PATCH 17/27] ext4: implement writeback iomap path

Zhang Yi
2024-10-22T03:13:02Z

From: Zhang Yi <yi.zhang@huawei.com>

Implement ext4_iomap_writepages(), introduce ext4_writeback_ops, and
create an end I/O extent conversion worker to implement the iomap
buffered write-back path. In the map_blocks() handler, we first query
the longest range of existing mapped extents. If the block range has not
already been allocated, we attempt to allocate a range of blocks that is
as long as possible to minimize the number of block mappings. This
allocation is based on the write-back length and the delalloc extent
length, rather than allocating for a single folio at a time. In the
->prepare_ioend() handler, we register the end I/O worker to convert
unwritten extents into written extents.

There are three key differences between the buffer_head write-back path
and the iomap write-back path:

1) Since we aim to allocate a range of blocks as long as possible within
the writeback length for each invocation of ->map_blocks(), we may
allocate a long range but write less in certain corner cases.
Therefore, we cannot convert the extent to written in advance within
->map_blocks(). Fortunately, there is minimal risk of losing data
between split extents during the write-back and the end I/O process.
We defer this action to the end I/O worker, where we can accurately
determine the actual written length. Besides, we should remove the
warning in ext4_convert_unwritten_extents_endio().
2) Since we do not order data, the journal thread is not required to
write back data. Besides, we also do not need to use the reserve
handle when converting the unwritten extent in the end I/O worker, we
can start normal handle directly.
3) We can also delay updating the i_disksize until the end of the I/O,
which could prevent the exposure of zero data that may occur during a
system crash while performing buffer append writes in the buffer_head
buffered write path.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
fs/ext4/ext4.h | 4 +
fs/ext4/extents.c | 22 +++---
fs/ext4/inode.c | 188 +++++++++++++++++++++++++++++++++++++++++++++-
fs/ext4/page-io.c | 105 ++++++++++++++++++++++++++
fs/ext4/super.c | 2 +
5 files changed, 311 insertions(+), 10 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index a09f96ef17d8..d4d594d97634 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1151,6 +1151,8 @@ struct ext4_inode_info {
*/
struct list_head i_rsv_conversion_list;
struct work_struct i_rsv_conversion_work;
+ struct list_head i_iomap_ioend_list;
+ struct work_struct i_iomap_ioend_work;

spinlock_t i_block_reservation_lock;

@@ -3773,6 +3775,8 @@ int ext4_bio_write_folio(struct ext4_io_submit *io, struct folio *page,
size_t len);
extern struct ext4_io_end_vec *ext4_alloc_io_end_vec(ext4_io_end_t *io_end);
extern struct ext4_io_end_vec *ext4_last_io_end_vec(ext4_io_end_t *io_end);
+extern void ext4_iomap_end_io(struct work_struct *work);
+extern void ext4_iomap_end_bio(struct bio *bio);

/* mmp.c */
extern int ext4_multi_mount_protect(struct super_block *, ext4_fsblk_t);
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 33bc2cc5aff4..4b30e6f0a634 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3760,20 +3760,24 @@ ext4_convert_unwritten_extents_endio(handle_t *handle, struct inode *inode,
ext_debug(inode, "logical block %llu, max_blocks %u\n",
(unsigned long long)ee_block, ee_len);

- /* If extent is larger than requested it is a clear sign that we still
- * have some extent state machine issues left. So extent_split is still
- * required.
- * TODO: Once all related issues will be fixed this situation should be
- * illegal.
+ /*
+ * If the extent is larger than requested, we should split it here.
+ * For inodes using the iomap buffered I/O path, we do not split in
+ * advance during the write-back process. Therefore, we may need to
+ * perform the split during the end I/O process here. However,
+ * other inodes should not require this action.
*/
if (ee_block != map->m_lblk || ee_len > map->m_len) {
int flags = EXT4_GET_BLOCKS_CONVERT |
EXT4_GET_BLOCKS_METADATA_NOFAIL;
#ifdef CONFIG_EXT4_DEBUG
- ext4_warning(inode->i_sb, "Inode (%ld) finished: extent logical block %llu,"
- " len %u; IO logical block %llu, len %u",
- inode->i_ino, (unsigned long long)ee_block, ee_len,
- (unsigned long long)map->m_lblk, map->m_len);
+ if (!ext4_test_inode_state(inode, EXT4_STATE_BUFFERED_IOMAP)) {
+ ext4_warning(inode->i_sb,
+ "Inode (%ld) finished: extent logical block %llu, len %u; IO logical block %llu, len %u",
+ inode->i_ino, (unsigned long long)ee_block,
+ ee_len, (unsigned long long)map->m_lblk,
+ map->m_len);
+ }
#endif
path = ext4_split_convert_extents(handle, inode, map, path,
flags, NULL);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 23cbcaab0a56..a260942fd2dd 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -44,6 +44,7 @@
#include <linux/iversion.h>

#include "ext4_jbd2.h"
+#include "ext4_extents.h"
#include "xattr.h"
#include "acl.h"
#include "truncate.h"
@@ -3710,10 +3711,195 @@ static void ext4_iomap_readahead(struct readahead_control *rac)
iomap_readahead(rac, &ext4_iomap_buffered_read_ops);
}

+struct ext4_writeback_ctx {
+ struct iomap_writepage_ctx ctx;
+ struct writeback_control *wbc;
+ unsigned int data_seq;
+};
+
+static int ext4_iomap_map_one_extent(struct inode *inode,
+ struct ext4_map_blocks *map)
+{
+ struct extent_status es;
+ handle_t *handle = NULL;
+ int credits, map_flags;
+ int retval;
+
+ credits = ext4_da_writepages_trans_blocks(inode);
+ handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, credits);
+ if (IS_ERR(handle))
+ return PTR_ERR(handle);
+
+ map->m_flags = 0;
+ /*
+ * It is necessary to look up extent and map blocks under i_data_sem
+ * in write mode, otherwise, the delalloc extent may become stale
+ * during concurrent truncate operations.
+ */
+ down_write(&EXT4_I(inode)->i_data_sem);
+ if (likely(ext4_es_lookup_extent(inode, map->m_lblk, NULL, &es))) {
+ retval = es.es_len - (map->m_lblk - es.es_lblk);
+ map->m_len = min_t(unsigned int, retval, map->m_len);
+
+ if (ext4_es_is_delayed(&es)) {
+ map->m_flags |= EXT4_MAP_DELAYED;
+ trace_ext4_da_write_pages_extent(inode, map);
+ /*
+ * Call ext4_map_create_blocks() to allocate any
+ * delayed allocation blocks. It is possible that
+ * we're going to need more metadata blocks, however
+ * we must not fail because we're in writeback and
+ * there is nothing we can do so it might result in
+ * data loss. So use reserved blocks to allocate
+ * metadata if possible.
+ */
+ map_flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT |
+ EXT4_GET_BLOCKS_METADATA_NOFAIL;
+
+ retval = ext4_map_create_blocks(handle, inode, map,
+ map_flags);
+ goto out;
+ }
+ if (unlikely(ext4_es_is_hole(&es)))
+ goto out;
+
+ /* Found written or unwritten extent. */
+ map->m_pblk = ext4_es_pblock(&es) + map->m_lblk -
+ es.es_lblk;
+ map->m_flags = ext4_es_is_written(&es) ?
+ EXT4_MAP_MAPPED : EXT4_MAP_UNWRITTEN;
+ goto out;
+ }
+
+ retval = ext4_map_query_blocks(handle, inode, map);
+out:
+ up_write(&EXT4_I(inode)->i_data_sem);
+ ext4_journal_stop(handle);
+ return retval < 0 ? retval : 0;
+}
+
+static int ext4_iomap_map_blocks(struct iomap_writepage_ctx *wpc,
+ struct inode *inode, loff_t offset,
+ unsigned int dirty_len)
+{
+ struct ext4_writeback_ctx *ewpc =
+ container_of(wpc, struct ext4_writeback_ctx, ctx);
+ struct super_block *sb = inode->i_sb;
+ struct journal_s *journal = EXT4_SB(sb)->s_journal;
+ struct ext4_inode_info *ei = EXT4_I(inode);
+ struct ext4_map_blocks map;
+ unsigned int blkbits = inode->i_blkbits;
+ unsigned int index = offset >> blkbits;
+ unsigned int end, len;
+ int ret;
+
+ if (unlikely(ext4_forced_shutdown(inode->i_sb)))
+ return -EIO;
+
+ /* Check validity of the cached writeback mapping. */
+ if (offset >= wpc->iomap.offset &&
+ offset < wpc->iomap.offset + wpc->iomap.length &&
+ ewpc->data_seq == READ_ONCE(ei->i_es_seq))
+ return 0;
+
+ end = min_t(unsigned int, (ewpc->wbc->range_end >> blkbits),
+ (UINT_MAX - 1));
+ len = (end > index + dirty_len) ? end - index + 1 : dirty_len;
+
+retry:
+ map.m_lblk = index;
+ map.m_len = min_t(unsigned int, MAX_WRITEPAGES_EXTENT_LEN, len);
+ ret = ext4_map_blocks(NULL, inode, &map, 0);
+ if (ret < 0)
+ return ret;
+
+ /*
+ * The map is not a delalloc extent, it must either be a hole
+ * or an extent which have already been allocated.
+ */
+ if (!(map.m_flags & EXT4_MAP_DELAYED))
+ goto out;
+
+ /* Map one delalloc extent. */
+ ret = ext4_iomap_map_one_extent(inode, &map);
+ if (ret < 0) {
+ if (ext4_forced_shutdown(sb))
+ return ret;
+
+ /*
+ * Retry transient ENOSPC errors, if
+ * ext4_count_free_blocks() is non-zero, a commit
+ * should free up blocks.
+ */
+ if (ret == -ENOSPC && journal && ext4_count_free_clusters(sb)) {
+ jbd2_journal_force_commit_nested(journal);
+ goto retry;
+ }
+
+ ext4_msg(sb, KERN_CRIT,
+ "Delayed block allocation failed for inode %lu at logical offset %llu with max blocks %u with error %d",
+ inode->i_ino, (unsigned long long)map.m_lblk,
+ (unsigned int)map.m_len, -ret);
+ ext4_msg(sb, KERN_CRIT,
+ "This should not happen!! Data will be lost\n");
+ if (ret == -ENOSPC)
+ ext4_print_free_blocks(inode);
+ return ret;
+ }
+out:
+ ewpc->data_seq = READ_ONCE(ei->i_es_seq);
+ ext4_set_iomap(inode, &wpc->iomap, &map, offset,
+ map.m_len << blkbits, 0);
+ return 0;
+}
+
+static int ext4_iomap_prepare_ioend(struct iomap_ioend *ioend, int status)
+{
+ struct ext4_inode_info *ei = EXT4_I(ioend->io_inode);
+
+ /* Need to convert unwritten extents when I/Os are completed. */
+ if (ioend->io_type == IOMAP_UNWRITTEN ||
+ ioend->io_offset + ioend->io_size > READ_ONCE(ei->i_disksize))
+ ioend->io_bio.bi_end_io = ext4_iomap_end_bio;
+
+ return status;
+}
+
+static void ext4_iomap_discard_folio(struct folio *folio, loff_t pos)
+{
+ struct inode *inode = folio->mapping->host;
+ loff_t length = folio_pos(folio) + folio_size(folio) - pos;
+
+ ext4_iomap_punch_delalloc(inode, pos, length, NULL);
+}
+
+static const struct iomap_writeback_ops ext4_writeback_ops = {
+ .map_blocks = ext4_iomap_map_blocks,
+ .prepare_ioend = ext4_iomap_prepare_ioend,
+ .discard_folio = ext4_iomap_discard_folio,
+};
+
static int ext4_iomap_writepages(struct address_space *mapping,
struct writeback_control *wbc)
{
- return 0;
+ struct inode *inode = mapping->host;
+ struct super_block *sb = inode->i_sb;
+ long nr = wbc->nr_to_write;
+ int alloc_ctx, ret;
+ struct ext4_writeback_ctx ewpc = {
+ .wbc = wbc,
+ };
+
+ if (unlikely(ext4_forced_shutdown(sb)))
+ return -EIO;
+
+ alloc_ctx = ext4_writepages_down_read(sb);
+ trace_ext4_writepages(inode, wbc);
+ ret = iomap_writepages(mapping, wbc, &ewpc.ctx, &ext4_writeback_ops);
+ trace_ext4_writepages_result(inode, wbc, ret, nr - wbc->nr_to_write);
+ ext4_writepages_up_read(sb, alloc_ctx);
+
+ return ret;
}

/*
diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index ad5543866d21..659ee0fb7cea 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -22,6 +22,7 @@
#include <linux/bio.h>
#include <linux/workqueue.h>
#include <linux/kernel.h>
+#include <linux/iomap.h>
#include <linux/slab.h>
#include <linux/mm.h>
#include <linux/sched/mm.h>
@@ -562,3 +563,107 @@ int ext4_bio_write_folio(struct ext4_io_submit *io, struct folio *folio,

return 0;
}
+
+static void ext4_iomap_finish_ioend(struct iomap_ioend *ioend)
+{
+ struct inode *inode = ioend->io_inode;
+ struct ext4_inode_info *ei = EXT4_I(inode);
+ loff_t pos = ioend->io_offset;
+ size_t size = ioend->io_size;
+ loff_t new_disksize;
+ handle_t *handle;
+ int credits;
+ int ret, err;
+
+ ret = blk_status_to_errno(ioend->io_bio.bi_status);
+ if (unlikely(ret))
+ goto out;
+
+ /*
+ * We may need to convert up to one extent per block in
+ * the page and we may dirty the inode.
+ */
+ credits = ext4_chunk_trans_blocks(inode,
+ EXT4_MAX_BLOCKS(size, pos, inode->i_blkbits));
+ handle = ext4_journal_start(inode, EXT4_HT_EXT_CONVERT, credits);
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out_err;
+ }
+
+ if (ioend->io_type == IOMAP_UNWRITTEN) {
+ ret = ext4_convert_unwritten_extents(handle, inode, pos, size);
+ if (ret)
+ goto out_journal;
+ }
+
+ /*
+ * Update on-disk size after IO is completed. Races with
+ * truncate are avoided by checking i_size under i_data_sem.
+ */
+ new_disksize = pos + size;
+ if (new_disksize > READ_ONCE(ei->i_disksize)) {
+ down_write(&ei->i_data_sem);
+ new_disksize = min(new_disksize, i_size_read(inode));
+ if (new_disksize > ei->i_disksize)
+ ei->i_disksize = new_disksize;
+ up_write(&ei->i_data_sem);
+ ret = ext4_mark_inode_dirty(handle, inode);
+ if (ret)
+ EXT4_ERROR_INODE_ERR(inode, -ret,
+ "Failed to mark inode dirty");
+ }
+
+out_journal:
+ err = ext4_journal_stop(handle);
+ if (!ret)
+ ret = err;
+out_err:
+ if (ret < 0 && !ext4_forced_shutdown(inode->i_sb)) {
+ ext4_msg(inode->i_sb, KERN_EMERG,
+ "failed to convert unwritten extents to written extents or update inode size -- potential data loss! (inode %lu, error %d)",
+ inode->i_ino, ret);
+ }
+out:
+ iomap_finish_ioends(ioend, ret);
+}
+
+/*
+ * Work on buffered iomap completed IO, to convert unwritten extents to
+ * mapped extents
+ */
+void ext4_iomap_end_io(struct work_struct *work)
+{
+ struct ext4_inode_info *ei = container_of(work, struct ext4_inode_info,
+ i_iomap_ioend_work);
+ struct iomap_ioend *ioend;
+ struct list_head ioend_list;
+ unsigned long flags;
+
+ spin_lock_irqsave(&ei->i_completed_io_lock, flags);
+ list_replace_init(&ei->i_iomap_ioend_list, &ioend_list);
+ spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
+
+ iomap_sort_ioends(&ioend_list);
+ while (!list_empty(&ioend_list)) {
+ ioend = list_entry(ioend_list.next, struct iomap_ioend, io_list);
+ list_del_init(&ioend->io_list);
+ iomap_ioend_try_merge(ioend, &ioend_list);
+ ext4_iomap_finish_ioend(ioend);
+ }
+}
+
+void ext4_iomap_end_bio(struct bio *bio)
+{
+ struct iomap_ioend *ioend = iomap_ioend_from_bio(bio);
+ struct ext4_inode_info *ei = EXT4_I(ioend->io_inode);
+ struct ext4_sb_info *sbi = EXT4_SB(ioend->io_inode->i_sb);
+ unsigned long flags;
+
+ /* Only reserved conversions from writeback should enter here */
+ spin_lock_irqsave(&ei->i_completed_io_lock, flags);
+ if (list_empty(&ei->i_iomap_ioend_list))
+ queue_work(sbi->rsv_conversion_wq, &ei->i_iomap_ioend_work);
+ list_add_tail(&ioend->io_list, &ei->i_iomap_ioend_list);
+ spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
+}
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index a01e0bbe57c8..56baadec27e0 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1419,11 +1419,13 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
#endif
ei->jinode = NULL;
INIT_LIST_HEAD(&ei->i_rsv_conversion_list);
+ INIT_LIST_HEAD(&ei->i_iomap_ioend_list);
spin_lock_init(&ei->i_completed_io_lock);
ei->i_sync_tid = 0;
ei->i_datasync_tid = 0;
atomic_set(&ei->i_unwritten, 0);
INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
+ INIT_WORK(&ei->i_iomap_ioend_work, ext4_iomap_end_io);
ext4_fc_init_inode(&ei->vfs_inode);
mutex_init(&ei->i_fc_lock);
return &ei->vfs_inode;
--
2.46.1